The activated partial thromboplastin time (aPTT) is a blood test that measures the intrinsic and common pathways of the coagulation cascade, primarily assessing the function of factors VIII, IX, XI, XII, and the contact activation system, as well as the common pathway (factors I, II, V, X). It is most commonly used to monitor unfractionated heparin therapy, to screen for bleeding disorders (e.g., hemophilia A/B, von Willebrand disease), to detect lupus anticoagulant, or to evaluate prolonged clotting in disseminated intravascular coagulation (DIC). Normal aPTT range is typically 25–35 seconds (lab-specific), and prolongation indicates deficiency or inhibition of the intrinsic pathway factors, while a shortened aPTT may suggest hypercoagulability.

🧪 Activated Partial Thromboplastin Time (APTT)

Measures the activity of the intrinsic and common pathways of coagulation.
Normal range: 27–35 seconds ⏱️
Used to investigate unexplained bleeding or clotting.
Often ordered alongside Prothrombin Time (PT) to evaluate haemostasis.
Assesses factors XII, XI, IX, VIII, X, V, II (prothrombin), I (fibrinogen), plus prekallikrein (PK) and high-molecular-weight kininogen (HK).

⏳ Causes of an Isolated Prolonged APTT

💉 Unfractionated Heparin therapy (not LMWH/danaparoid).
🧬 Haemophilia A (FVIII) or Haemophilia B (FIX).
🧪 Antiphospholipid syndrome (lupus anticoagulant).
🦠 Haemolytic uraemic syndrome.
🦠Von Willebrand’s disease.
💊 Rivaroxaban, Dabigatran (DOACs).
🦠Inherited deficiencies of XII, XI, IX, VIII.

⚡ Causes of a Shortened APTT

Acute phase response → ↑ Factor VIII.
Sample collection errors → in-tube clot activation.

📊 Interpretation of PT + APTT Combinations

📉 Prolonged PT + Normal APTT → Liver disease, vitamin K deficiency, or Factor VII defect.
📉 Normal PT + Prolonged APTT → Haemophilia A/B, Factor XI or XII deficiency, Von Willebrand disease, lupus anticoagulant.
📉 Prolonged PT + Prolonged APTT → Severe liver disease, DIC, or Factor I/II/V/X deficiencies.
✅ Normal PT + Normal APTT → Usually normal haemostasis, but can miss mild deficiencies or mild Von Willebrand disease → further testing may be needed.

💡 Exam tip: – PT checks the extrinsic pathway (Factor VII). – APTT checks the intrinsic pathway (Factors VIII, IX, XI, XII). – Both prolonged → think liver disease, DIC, or multiple factor deficiency.

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APTT and Coagulation

Introduction

🧪 Activated Partial Thromboplastin Time (APTT)

⏳ Causes of an Isolated Prolonged APTT

⚡ Causes of a Shortened APTT

📊 Interpretation of PT + APTT Combinations

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